Public bug reported:

== SRU Justification ==

[Impact]
Oops during heavy NFS + FSCache + Cachefiles use:

 CacheFiles: Error: Overlong wait for old active object to go away.
 BUG: unable to handle kernel NULL pointer dereference at 0000000000000002

 CacheFiles: Error: Object already active
 kernel BUG at fs/cachefiles/namei.c:163!

[Cause]
  In a heavily loaded system with big files being read and truncated,
  an fscache object for a cookie is being dropped and a new object being looked.
  The new object being looked for has to wait for the old object to go away 
before the
  new object is moved to active state.

[Fix]
     Clear the flag 'CACHEFILES_OBJECT_ACTIVE' for the new object when retrying 
     the object lookup.
     Remove the BUG() for the case where the old object is still being dropped
     and convert to WARN()

[Testcase]
A user has run ~100 hours of NFS stress tests and not seen this bug recur.

[Regression Potential]
 - Limited to fscache/cachefiles.

** Affects: linux (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1776254

Title:
  CacheFiles: Error: Overlong wait for old active object to go away.

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